When writing ReRAM cells, it is pursued to set the cells in a sufficiently high or low resistance state, while preventing excessive writing. Disclosed is a semiconductor storage device including memory cells, each including a variable resistance element, and control circuitry that executes an Off writing process of applying Off writing pulse to a memory cell to turn it into high resistance state and an On writing process of applying On writing pulse to turn it into low resistance state. The control circuitry, when the memory cell is placed in low resistance state, after applying Off writing pulse, applies a reading pulse for a verify process of reading whether it is placed in high or low resistance state. If the memory cell is not placed in high resistance state as a result of the verify process, the control circuitry applies a reset pulse comprising On writing pulse, applies Off writing pulse with extended pulse width and executes the verify process in mentioned order.
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15. A semiconductor device, comprising:
a memory cell; and
control circuitry that executes a first writing process of applying a first writing pulse to the memory cell to turn the memory cell state into a first resistance state in which the resistance value of the variable resistance element meets a first criterion and a second writing process of applying a second writing pulse of opposite polarity to the first writing pulse to turn the memory cell into a second resistance state meeting a second criterion,
wherein the control circuitry, when the memory cell is placed in the second resistance state, after applying the first writing pulse to the memory cell, applies a reading pulse for a verify process of reading whether the variable resistance element is placed in the first resistance state or the second resistance state, and
wherein, when the memory cell is not placed in the first resistance state as a result of the verify process, the control circuitry applies a reset pulse comprising the second writing pulse of a constant width to the memory cell, applies the first writing pulse with a pulse width made longer than the first writing pulse applied last, and executes the verify process.
20. A method of a semiconductor storage device, the method comprising:
executing, by a control circuitry, a first writing process of applying a first writing pulse to a memory cell to turn the memory cell state into a first resistance state in which the resistance value of the variable resistance element meets a first criterion and a second writing process of applying a second writing pulse of opposite polarity to the first writing pulse to turn the memory cell into a second resistance state meeting a second criterion;
applying, by the control circuitry, a reading pulse for a verify process of reading whether the variable resistance element is placed in the first resistance state or the second resistance state when the memory cell is placed in the second resistance state, after applying the first writing pulse to the memory cell;
when the memory cell is not placed in the first resistance state as a result of the verify process, applying by the control circuitry, a reset pulse comprising the second writing pulse of a constant width to the memory cell;
applying the first writing pulse with a pulse width made longer than the first writing pulse applied last; and
executing the verify process again.
1. A semiconductor storage device comprising:
memory cells, each comprising a variable resistance element; and
control circuitry that executes a first writing process of applying a first writing pulse to a memory cell to turn the memory cell state into a first resistance state in which the resistance value of the variable resistance element meets a first criterion and a second writing process of applying a second writing pulse of opposite polarity to the first writing pulse to turn the memory cell into a second resistance state meeting a second criterion,
wherein the control circuitry, when the memory cell is placed in the second resistance state, after applying the first writing pulse to the memory cell, applies a reading pulse for a verify process of reading whether the variable resistance element is placed in the first resistance state or the second resistance state, and
wherein, if the memory cell is not placed in the first resistance state as a result of the verify process, the control circuitry applies a reset pulse comprising the second writing pulse of a constant width to the memory cell, applies the first writing pulse with a pulse width made longer than the first writing pulse applied last, and executes the verify process in mentioned order.
2. The semiconductor storage device according to
wherein the control circuitry applies a pulse that is weaker than the second writing pulse as the reset pulse.
3. The semiconductor storage device according to
wherein the control circuitry applies a pulse whose voltage amplitude is smaller than the second writing pulse as the reset pulse.
4. The semiconductor storage device according to
wherein the control circuitry, when applying the reset pulse, causes a current for writing to be smaller than when applying the second writing pulse.
5. The semiconductor storage device according to
wherein the control circuitry applies a pulse with a shorter pulse width than the second writing pulse as the reset pulse.
6. The semiconductor storage device according to
wherein, if the memory cell is not placed in the first resistance state as a result of the verify process, the control circuitry extends pulse width by adding a predetermined value to the pulse width of the first writing pulse applied last.
7. The semiconductor storage device according to
Wherein, if the memory cell is not placed in the first resistance state as a result of the verify process, the control circuitry extends pulse width by multiplying the pulse width of the first writing pulse applied last by a predetermined value.
8. The semiconductor storage device according to
wherein, if the memory cell is not placed in the first resistance state as a result of the verify process, the control circuitry repeats applying a reset pulse comprising the second writing pulse to the memory cell, applying the same pulse as the first writing pulse applied last, and executing the verify process in mentioned order by a predetermined number of times, and even after that, if the first writing is unsuccessful, the control circuitry applies the first writing pulse with a pulse width made longer than the first writing pulse applied last, followed by executing the verify process.
9. The semiconductor storage device according to
wherein the first resistance state is a state in which the resistance value of the variable resistance element is equal to or more than a first criterion value and the second resistance state is a state in which the resistance value of the variable resistance element is less than a second criterion value.
10. The semiconductor storage device according to
wherein the first resistance state is a state in which the resistance value of the variable resistance element is less than the first criterion value and the second resistance state is a state in which the resistance value of the variable resistance element is equal to or more than the second criterion value.
11. The semiconductor storage device according to
wherein, if the memory cell is not placed in the first resistance state as a result of the verify process, the control circuitry applies a reset pulse comprising the second writing pulse to the memory cell, applies the first writing pulse more times than when it last applied the first writing pulse, and executes the verify process in mentioned order.
12. The semiconductor storage device according to
wherein, if the memory cell is not placed in the first resistance state even after having repeated the first writing pulse application and the verify process by a predetermined number of times, the control circuitry increases the amplitude of the voltage of the first writing pulse and initializes the pulse width of the first writing pulse and then repeats the first writing pulse application and the verify process.
13. The semiconductor storage device according to
14. The semiconductor storage device according to
16. The semiconductor storage device according to
wherein the control circuitry applies a pulse that is weaker than the second writing pulse as the reset pulse.
17. The semiconductor storage device according to
wherein the control circuitry applies a pulse whose voltage amplitude is less than the second writing pulse as the reset pulse.
18. The semiconductor storage device according to
wherein the control circuitry, when applying the reset pulse, causes a current for writing to be less than when applying the second writing pulse.
19. The semiconductor storage device according to
wherein the control circuitry applies a pulse with a shorter pulse width than the second writing pulse as the reset pulse, and
wherein, when the memory cell is not placed in the first resistance state as a result of the verify process, the control circuitry extends pulse width by adding a predetermined value to the pulse width of the first writing pulse applied last for each successive cycle.
21. The semiconductor storage device according to
wherein, when the memory cell is determined to not be in the first resistance state as the result of the verify process, the control circuitry applies the reset pulse comprising the second writing pulse of the constant width for each cycle to the memory cell, and then applies the first writing pulse with the pulse width made longer by a certain pattern increase with each cycle than the first writing pulse applied last.
22. The semiconductor storage device according to
wherein a width and a voltage of the second writing pulse of the opposite polarity is kept constant, and
wherein the first writing pulse with the pulse width made longer is an OFF writing pulse.
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The disclosure of Japanese Patent Application No. 2015-051855 filed on Mar. 16, 2015 including the specification, drawings and abstract is incorporated herein by reference in its entirety.
The present invention relates to a semiconductor storage device and, particularly, resides in a technique that is applicable to a semiconductor device using variable resistance elements.
Patent Document 1 (Japanese Published Unexamined Patent Application No. 2013-200922) describes a verify writing method that, when setting a cell of a Resistance Random Access Memory (ReRAM) in a low resistance state, if the resistance value of the cell has not reached a desired value after the execution of applying a writing pulse and verify reading, repeats a series of processing operations as follows: applying a pulse of opposite polarity→applying a rewriting pulse→verify reading. Patent Document 2 (Japanese Published Unexamined Patent Application No. Hei 06(1994)-60674) and Patent Document 3 (Japanese Published Unexamined Patent Application No. 2005-44454 respectively describe verify writing methods that execute verify writing, while increasing the time during which a writing pulse of the same polarity is applied.
The methods described in P Japanese Published Unexamined Patent Application No. Hei 06(1994)-60674 and Japanese Published Unexamined Patent Application No. 2005-44454, when applied to ReRAM, have a problem below: once a ReRAM cell has been set in an intermediate resistance state, i.e., neither high nor low resistance state, a sufficient current or voltage necessary for switching is not applied even by repeatedly applying a writing pulse of the same polarity, which may make it hard to change the resistance state.
On the other hand, the method described in Japanese Published Unexamined Patent Application No. 2013-200922 can cope with the above problem by applying a pulse of opposite polarity after very reading. However, when gradually increasing the voltage and pulse width of the writing pulse (or when keeping them constant), the above method is to gradually increase the voltage and pulse width of the pulse of opposite polarity likewise (or keep them constant) accordingly. Therefore, it is impossible for this method to sufficiently exert an effect that is obtained by gradually increasing the voltage and pulse width of the writing pulse in the process of attempts of repeating writing. That is, this method has a problem below: it is impossible to sufficiently exert a verify effect in which, when writing a memory cell to set it in a low resistance state, writing under optimal conditions is accomplished by repeating attempts, while gradually changing balance with writing of opposite polarity and changing conditions.
Furthermore, the above document only describes operations for writing a memory cell to set it in a low resistance state (On writing), but there is no description about whether such method is applicable to writing a memory cell to set it in a high resistance state (Off writing) among others.
Other problems and novel features will become apparent from the description in the present specification and the accompanying drawings.
A semiconductor storage device according to one embodiment includes memory cells, each including a variable resistance element, and control circuitry that executes a first writing process of applying a first writing pulse to the memory cell to turn the memory cell state into a first resistance state in which the resistance value of the variable resistance element meets a first criterion and a second writing process of applying a second writing pulse of opposite polarity to the first writing pulse to turn the memory cell into a second resistance state meeting a second criterion.
The control circuitry, when the memory cell is placed in the second resistance state, after applying the first writing pulse to the memory cell, applies a reading pulse for a verify process of reading whether the variable resistance element is placed in the first resistance state or the second resistance state. If the memory cell is not placed in the first resistance state as a result of the verify process, the control circuitry applies a reset pulse formed of the second writing pulse to the memory cell, applies the first writing pulse with a pulse width made longer than the first writing pulse applied last, and executes the verify process in mentioned order.
According to the above one embodiment, it is possible to set ReRAM cells in a sufficiently high or low resistance state, while preventing excessive writing.
In the following, embodiments of the invention will be described in detail based on the drawings. In all the drawings for describing the embodiments, identical components are assigned the same referential numbers or marks and their description is not repeated.
Because of, inter alia, characteristic variation of resistance elements of ReRAM, ReRAM cells corresponding to bits may have a too high or too low value as a resistance value (Off resistance) in a high resistance state (Off state). For cells which have a too low value of Off resistance, a method of extending the width of a pulse for Off writing can be taken as one method for characteristic improvement to make their resistance value as high as possible. However, extending the pulse width may make the Off resistance value too high, which may delay switching to a low resistance state (On state) during On writing which is performed subsequently or, in some instances, may result in failure of On writing for some bits.
Therefore, in embodiments which will be described hereinafter, as a method for optimizing Off writing so that ReRAM cells will have sufficient Off resistance, while preventing excessive writing as noted above, a pulse of opposite direction is once applied before rewriting in a process of verify rewriting in which the pulse width is increased gradually during Off writing. In this process, balance when Off writing is executed is changed appropriately by keeping the pulse width and voltage of the pulse of opposite direction constant, thereby making it possible to perform optimal Off writing. Embodiments which will be described hereinafter do not apply only to Off writing; these embodiments are also applicable to On writing in the same way, unless otherwise stated.
The variable resistance layer VRL is made of, e.g., metallic oxide (for example, a tantalic oxide, titanium oxide, zirconium oxide, or hafnium oxide). The variable resistance layer VRL of this instance may be a monolayer film or laminated film. Supposing that the variable resistance layer VRL is a laminated film, the variable resistance layer VRL may be a laminated film with layers having different combinations of types of elements or a laminated film with layers having the same combination of types of elements, where the oxygen composition ratio of each layer differs. The film thickness of the variable resistance layer VRL is, e.g., between 1.5 nm to 30 nm inclusive. Each of the metal layers M1 and M2 is made of, e.g., ruthenium, titanium nitride, tantalum, tantalum nitride, tungsten, palladium, or platinum.
One terminal of the variable resistance element VR is coupled to the plate line PL and the other terminal thereof is coupled via the selection transistor TR to the bit line BL. A gate of the selection transistor TR is coupled to a word line WL. By making either the potential of the bit line BL or the potential of the plate line PL higher than the other, the polarity of the variable resistance element VR can be switched over.
Although it is not determinative whether the metal layer M1 or the metal layer M2 is coupled to the plate line PL, descriptions will be provided below, assuming that the metal layer M2 is coupled to the plate line PL. Although it is not determinative whether the selection transistor TR should be N-channel type or P-channel type, descriptions will be provided below, assuming that the transistor is N-channel type in which source-drain conduction occurs by applying a positive voltage to the gate. In the case of a P-channel type transistor, source-drain conduction occurs by applying a negative voltage to the gate.
Meanwhile, reading a resistance state is performed by applying a positive voltage for reading (VR which is larger than Von) to the end coupled to the plate line PL and detecting a current flowing through the VR without changing the resistance state of the variable resistance element VR.
The memory cells MC each are coupled to nodes where word lines WL0 to WL3 intersect with bit lines BL0 to BL3 and plate lines PL0 to PL3. All the word lines WL0 to WL3, bit lines BL0 to BL3, and plate lines PL0 to PL3 are coupled to control circuits, not depicted, in the periphery of the memory cell array MCA. For example, the word lines WL0 to WL3 are coupled to a word line control circuit, not depicted, on the left side in the drawing of the memory cell array MCA. The bit lines BL0 to BL3 are coupled to a bit line control circuit, not depicted, on the upper side in the drawing. Likewise, the plate lines PL0 to PL3 are coupled to a plate line control circuit, not depicted, on the upper side in the drawing.
Control circuitry included of the above-mentioned respective control circuits writes a memory cell by applying a voltage to the appropriate word line WL, bit line BL, and plate line PL, thus turning the desired memory cell MC into a high or low resistance state. The control circuitry reads a memory cell by detecting a current flowing through the appropriate bit line BL or plate line PL and deciding whether the desired memory cell MC is placed in a high or low resistance state.
To read whether the memory cell MC surrounded by a dotted circle is placed in an On state or Off state, word lines WL0, WL2, and WL3 and plate lines PL0, PL2, and PL3 other than the word line WL1 and the plate line PL1 and all bit lines BL0 to BL3 should be placed at a zero potential (GND) and the word line WL1 should be placed at a high potential (Vw3). Then, a voltage (VR) which is sufficiently lower than the voltage applied for writing should be applied to the plate line PL1 and a current flowing through the bit line BL1 or plate line PL1 should be detected.
During an operation described above, in memory cells MCs coupled to word lines other than the word line WL1, the selection transistors TR are non-conductive and no voltage is applied to the variable resistance elements VR. Besides, in memory cells MCs coupled to bit lines and plate lines other than the bit line BL1 and the plate line PL1, no voltage is applied to the variable resistance elements VR, since the bit lines BL0, BL2, and BL3 and the plate lines PL0, PL2, and PL3 are placed at the same potential. Hence, only the memory cell MC surrounded by a dotted circle is written or read. Writing and reading other memory cells MCs can be performed in the same way as described above.
Verify writing is performed as described below. After applying the Off writing pulse Poff, the control circuitry applies a reading pulse PR for verification and detects a current, thereby reading the resistance of the variable resistance element VR. Then, the control circuit decides whether or not writing is successful, according to the thus read value of resistance. If the value of resistance meets a predetermined criterion (the value is not less than a predetermined value), the control circuitry decides that the first time attempt is successful and terminates the writing process.
Otherwise, if the value of resistance of the variable resistance element VR does not meet the predetermined criterion (the value is less than the predetermined value), the control circuitry decides that the first time attempt is failed. After applying the Off writing pulse Poff again as a second time attempt, the control circuitry applies the reading pulse PR and reads the resistance of the variable resistance element VR, and decides whether writing is successful depending on whether the resistance value meets the predetermined criterion. If writing is yet unsuccessful, the control circuitry repeats attempts until writing by applying the Off writing pulse Poff is decided to be successful, for example, by a predetermined number of times set as a maximum.
In the present embodiment, for example, as depicted in
Otherwise, if the value of resistance of the variable resistance element VR does not meet the predetermined criterion (the value is less than the predetermined value), the control circuitry decides that the first time attempt is failed. The control circuitry applies an On writing pulse Pon with a pulse width ton (S05) to decrease the resistance of the variable resistance element VR once. Then, the control circuitry sets the pulse width of the Off writing pulse Poff to toff2 (which is larger than toff1) (S06). In this state, as a second time attempt, the control circuitry executes a series of processing operations as follows: Off writing, again (S02), verify reading (S03), and deciding whether writing is successful according to the resistance value (S04).
The control circuitry terminates the writing process, if having decided that Off writing is successful. But, if having decided that Off writing is failed, the control circuitry re-executes On writing (S05) with the On writing pulse Pon whose pulse width and voltage remain unchanged. After that, the control circuitry further increases the pulse width to toff3 which is larger than toff2 (S06) and executes verify writing (S02 thru S04). The control circuitry repeats a series of processing operations described above until Off writing is successful (the resistance value becomes equal to or more than the predetermined value).
The reason why the way of verify writing in which the pulse width of the Off writing pulse Poff is gradually increased is effective is as follows: because of characteristic variation of memory cells MC, each including a variable resistance element VR, rewriting energy required to set the memory cells MC in a high resistance state differs from one cell to another.
In the diagram, a dashed line depicts the characteristic of a memory cell MC that can be set in the high resistance state successfully with relatively small energy, indicating that the value of resistance of VR has reached the desired Off resistance (Rmin(Off)) during the application of the Off writing pulse Poff with a pulse width toff1. On the other hand, a solid line depicts the characteristic of a memory cell for which large energy is supposed to be needed to set it in the high resistance state, indicating that the application of the Off writing pulse Poff with a pulse width toff1 terminates when the resistance value has not yet reached the desired Off resistance (Rmin(Off)), since the value of resistance of VR starts to increase late. Besides, a dotted line in the diagram depicts how the MC behaves, assuming that the pulse width of the Off writing pulse Poff is longer than toff1; this shows that it takes longer until the MC is set in the high resistance state.
For such a memory cell MC, it is decided that rewriting it is needed. As depicted in
In the present embodiment, an On writing pulse for resetting the VR (reset pulse) is required to be the same pulse, every time the VR is reset. With a constant low resistance state (On state) created by applying a constant On writing pulse, the time during which the Off writing pulse is applied is made longer gradually. This can increase the probability in which the VR is set in the high resistance state (Off state), as attempts are repeated, and makes it possible to execute verify rewriting efficiently.
As in related art, for example, in a case of extending the On writing pulse width or increasing the On writing pulse voltage with an increase in the Off writing pulse width at each attempt, excessive resetting is performed; this changes the condition before Off writing is executed to a condition where the VR is hard to set in the Off state (more energy is required to increase the resistance of the VR). Consequently, the effect of extending the Off writing pulse width at each attempt is cancelled by excessive resetting, which may result in that the probability in which the VR is set in the Off state is hard to increase and verify rewriting cannot be executed efficiently.
Applying an Off writing pulse with a too long width to a memory cell MC for which Off writing is successful with an Off writing pulse with a short width may make a decrease in the success rate at a subsequent On writing. Hence, it is preferable to make the pulse width of an Off writing pulse as short as possible at the first time attempt to set the VR in the high resistance state with minimum necessary driving power. In particular, the pulse width can be set to, e.g., approx. 20 nsec. As the Off writing pulse width at a second time or subsequent attempt, a longer pulse width than in the preceding attempt can be selected by any method. For example, a method of incrementing the pulse width by a constant time in arithmetic progression and a method of incrementing the pulse width by a constant factor in geometric progression, among others, are conceivable.
The former method allows the pulse width to increase in small steps overall and, therefore, has a merit in which it enables detailed control in applying a writing pulse with a minimum necessary pulse width to each memory cell MC, whereas having a demerit in which, for a memory cell MC for which a long pulse width is required, verify rewriting it has to be repeated many times and, consequently, it may take long to write it. Conversely, the latter method enables verify writing even a memory cell MC for which a long pulse width is required by fewer times, that is, writing it for a relatively short period of time. On the other hand, an Off writing pulse with a too long pulse width may be applied to some of the memory cells MC. Therefore, in implementation, it is preferable to select an optimal method appropriately, depending on characteristic variation of actual variable resistance elements VR.
The example described above is an instance in which the Off writing pulse width is increased each time the Off writing pulse is applied once. Alternatively, it is also possible to repeat verify writing without changing the Off writing pulse width until writing has been attempted by a predetermined number of times and increment the pulse width, if writing is not performed properly, when attempted by the predetermined number of times.
For example, the control circuitry may apply an Off writing pulse with the same pulse width at the first and second time attempts and apply the Off writing pulse with a different pulse width at a third time or subsequent attempt. Increasing the pulse width may be adaptable such that, for example, the pulse width at the second time attempt is made longer than that at the first time attempt, the pulse width at the third time attempt is the same as that at the second time, and the pulse width at a fourth time attempt is made longer than that at the second and third time attempts. There is a possibility in which writing a bit that is almost fixed to a resistance state more than threshold resistance for writing may be succeeded by reattempt using the same pulse width. If doing so is possible, Off writing can be completed using minimum necessary energy for writing. Such adaptation in increasing the pulse width is not limited to the present embodiment and can also be applied to other embodiments likewise.
As presented in
Although an instance of Off writing has been described by way of example in the present embodiment, application to On writing is also possible.
More specifically, verify On writing with the pulse width being increased gradually can be performed, for example, as follows: in an inverse manner to control in Off writing, the bit line BL is replaced with the plate line PL as a signal line supplying a writing pulse and the control circuitry appropriately adjusts a voltage which is applied to the word line WL without changing the condition for applying the reading pulse PR for verification.
As set forth above, according to the ReRAM of the first embodiment, the control circuitry applies the On writing pulse Pon once before rewriting in the process of verify rewriting with the pulse width of the Off writing pulse Poff being increased gradually. Balance when Off writing is executed is changed appropriately by keeping the pulse width and voltage of the On writing pulse Pon constant. Thereby, it would become possible to set the ReRAM cells in a sufficiently high or low resistance state, while preventing excessive writing, and to perform optimal Off writing efficiently.
Although the reset pulse Prst is weakened by making the voltage of the reset pulse Prst lower than the voltage Von of the On writing pulse Pon (or the voltage Voff of the Off writing pulse Poff) in the present embodiment, the reset pulse Prst may be weakened by decreasing its pulse width.
Controlling the voltage to be applied to the word line WL is controlling the gate voltage of the selection transistor TR and is equivalent to controlling the current required to switch over the resistance state of the variable resistance element VR. Therefore, even by controlling the amount of current from a current source external to the memory cell array instead of the voltage to be applied to the word line, a configuration can be provided to obtain the same effect.
According to the ReRAM of the fourth embodiment, the control circuitry continuously applies the On writing pulse Pon with a minimum pulse width without increasing the pulse width of the On writing pulse Pon. It is not required to set several pulse widths of the On writing pulse Pon and the configuration can be simplified. Besides, the variable resistance element VR can be prevented from heating excessively and elaborative control can be performed, lessening variation of ON resistance values after the execution of On writing.
The address of a memory cell MC into which information will be written is first specified (step S11 in
If it is decided that writing has been attempted by the upper limit number of times k, but the resistance value does not reach the predetermined Off resistance value, the control circuitry sets the voltage of the Off writing pulse Poff to a higher voltage Voff2 (which is larger than Voff1), as presented in the middle row of
As described above, according to the ReRAM of the fifth embodiment, the control circuitry repeats a sequence of verify Off writing including a series of steps for gradually increasing the pulse width of the Off writing pulse Poff, while gradually increasing the voltage of the Off writing pulse Poff by each sequence, until a predetermined Off resistance value has been reached. Thereby, it is possible to prevent the pulse width from extending excessively and improve the efficiency of verify Off writing in the process of verify Off writing which is executed by gradually increasing the pulse width of the Off writing pulse Poff.
As a parameter other than the pulse width to be combined with verify Off writing which is executed by gradually increasing the pulse width of the Off writing pulse Poff, in addition to the amplitude of the voltage of the Off writing pulse Poff, for example, inter alia, a rewriting current required to switch over the resistance state of the variable resistance element VR can be used. The above rewriting current can be controlled by, e.g., the voltage applied to the word line WL or the current source external to the memory cell array MCA.
Although an instance of Off writing has been described by way of example in the present embodiment, application to On writing is also possible. For example, in an inverse manner to control in Off writing, the bit line BL is replaced with the plate line PL as a signal line supplying a writing pulse and the control circuitry appropriately adjusts a voltage which is applied to the word line WL without changing the condition for applying the reading pulse PR for verification. In this way, verify On writing with the pulse width being increased gradually can be implemented.
In the foregoing first through fifth embodiments, descriptions have been provided, taking, as an example, the structure in which a memory cell MC storing one bit of information is included of one variable resistance element VR and one selection transistor TR. The methods described in the embodiments, basically, can also be applied to a ReRAM of a so-called, cross-point type structure, except for a structure requiring a selection transistor TR, as set forth in the third embodiment.
The memory cells MC each are coupled to nodes where word lines WL0 to WL3 intersect with bit lines BL0 to BL3. All the word lines WL0 to WL3 and bit lines BL0 to BL3 are coupled to control circuits, not depicted, in the periphery of the memory cell array MCA. For example, the word lines WL0 to WL3 are coupled to a word line control circuit, not depicted, on the left side in the drawing of the memory cell array MCA. The bit lines BL0 to BL3 are coupled to a bit line control circuit, not depicted, on the upper side in the drawing.
Control circuitry included of the above-mentioned respective control circuits writes a memory cell by applying a voltage to the appropriate bit line and word line, thus turning the desired memory cell MC into a high or low resistance state. The control circuitry reads a memory cell by detecting a current flowing through the appropriate bit line or word line and deciding whether the desired memory cell is placed in a high or low resistance state.
For example, to write a memory cell MC surrounded by a dotted circle to turn it into an On state, a word line WL1 should be placed at a high potential, a bit line BL1 should be placed at a zero potential, and other word lines WL0, WL2, and WL3 and bit lines BL0, BL2, and BL3 should be placed at one half of the high potential. Inversely, to write the memory cell MC surrounded by a dotted circle to turn it into an OFF state, the word line WL1 should be placed at the zero potential, the bit line BL1 should be placed at a high potential, and other word lines WL0, WL2, and WL3 and bit lines BL0, BL2, and BL3, and other word lines WL0, WL2, and WL3 and bit lines BL0, BL2, and BL3 should be placed at one half of the high potential.
To read whether the memory cell MC surrounded by a dotted circle is placed in an On state or Off state, the bit line BL1 should be placed at the zero potential, other bit lines BL0, BL2, and BL3 and all word lines WL0 to WL3 should be placed at a high potential (which is, however, sufficiently lower than in writing), and a current flowing through the word line WL1 should be detected.
By an operation described above, the high potential is applied to both ends of only the memory cell MC coupled to the word line WL1 and the bit line BL1 and one half of the high potential or the zero potential is applied to other memory cells MCs. Hence, only the memory cell MC surrounded by a dotted circle is written or read. Writing and reading other memory cells MCs can be performed in the same way as described above.
The nonlinear resistance element NLR in the memory cell depicted in
Even to the cross-point type ReRAM described above, the verify writing methods set forth in the foregoing first, second, fourth, and fifth embodiments can be applied. That is, in the process of verify rewriting which is executed by gradually increasing the pulse width of the Off writing pulse Poff, by once applying the On writing pulse Pon with constant pulse width and voltage as a reset pulse before rewriting, it would become possible to perform optimal Off writing efficiently.
While the invention developed by the present inventors has been described specifically based on its embodiments hereinbefore, it goes without saying that the present invention is not limited to the foregoing embodiments and various modifications may be made thereto without departing from the scope of the invention.
Hase, Takashi, Furutake, Naoya, Masuzaki, Koji
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